The centrex service is a service offered by a teleoperator in addition to basic services. In the centrex service, a certain number of subscriber connections of an exchange are allocated to one organization, e.g., a company. The centrex service, which is associated with a telephone exchange and managed by an operator, offers the same services as a PBX or PABX exchange of a company, wherefore it is not necessary for so that a company need not purchase and maintain an exchange of its own. originally, only subscribers belonging to the same exchange could be included in the centrex service. The service thus allowed internal calls and call transfer from a subscriber line via an exchange only to another subscriber line connected to the same exchange. In the expanded service developed later, subscribers located geographically far from each other, and thus, subscribers connected to different exchanges, belonged to the same centrex group. This was possible only if the user of the service leased a private line which connected the different exchanges to each other. This is, however, expensive, and changes in the routing of a call require switching operations performed by the operator in the exchanges, which is cumbersome and costly.
The problem becomes more serious when a company has offices in many places over a geographically wide area, and the offices should be combined into one centrex group, whereby it is possible to make internal calls and call transfers through many exchanges.
A solution to this problem with regional centrex service is disclosed in PCT application WO 93/17515. It discloses a service whose architecture is adapted to an advanced intelligent network (AIN). In a network of this kind, an exchange is connected to one or more corresponding exchanges by trunk lines, and each exchange has a data link using common channel signaling to a signaling transfer point STP, which in turn is connected to an integrated service control point (ISPC).
The above system will be described in greater detail with reference to FIG. 1. Exchanges 11, 13, 15, 17, which are each indicated by SSP (Service Switching Point), are programmable exchanges. The exchanges identify AIN-type calls, send interrogations to ISCQ, and receive instructions and data therefrom for call processing. Exchanges 11, 13 are connected via data links to a regional signaling transfer point 23 (STP), and exchanges 15 and 17 are connected to another regional signaling transfer point. The exchanges are connected to each other by trunk lines, which transfer calls. Regional STP nodes 23 and 24 are connected to a regional STP node 31, which in turn is connected to a service control point ISCP 40. The connections from the exchanges to nodes 23 and 25 are data links, e.g., common channel signaling links according to CCITT #7, whereas the connections from the nodes to the STP node and from the STP node to the service point are packet network links. ISCP 40 comprises a service management system 41, a data and reporting system 45, and a service control point 43.
The regional centrex solution of the present invention is based on the idea that a centralized database is provided in the service point ISCP 40. This database contains all data relevant to the centrex service. The regional exchanges SSP 11, . . . , 17 are programmed to identify as regional centrex calls, the call and service requests from certain subscriber lines coupled to the service and connected to the exchange. When an exchange, e.g. exchange 11, receives a service request from such a subscriber line, e.g., terminal A, the exchange, first, detects that the line is a centrex line. Then, the exchange 11 sends a request on the signaling channel SS #7 through the signaling transfer points 23, 31 to the service control point ICP 40 for all information necessary to process the call. The request contains both the subnumber of the calling subscriber and the digits dialled by the subscriber. The control point 40 fetches the data of the calling subscriber from the database, and based on this data and the digits dialed gives the exchange 11 instructions for routing the call. The task may be, for example, a call or a call transfer to a subscriber D belonging to the same centrex group and connected to an exchange 17 which is geographically remotely situated.
In the convention system, centrex services are provided and changed, with regard to one subscriber line, in a centralized manner in this one service control point ISCP. The advantage of this system is that the centralization does not entail programming changes in exchanges controlled by it. The drawback of this system is, however, that its operational environment must always be an intelligent network (IN), which employs advanced signaling between specific signaling nodes SSP, STP, ICP, i.e., the system requires advanced signaling between the IN database center and the exchanges of both the calling and the called subscriber. The system is thus difficult to adapt to current telephone phone network systems, which have long been in use.
The object of the present invention is a regional centrex system which does not have the drawbacks of the known system and which is particularly suitable for use in current networks without being limited by possibly different signaling between the exchanges. The members of a closed user group must be able to establish speech connections using internal numbers of the group, i.e., subnumbers, which should be independent of the actual directory numbers of the subscribers. It must be possible to include subscribers from different exchanges and different local networks to a closed user group. It must be possible to identify subscribers belonging to the same closed user group and to offer them a number of different facilities. Addition of new numbers and removal of numbers, i.e., maintenance of the system, must be simple. Another object is to provide a system in which the operator can serve a plurality of centrex groups using the subnumbers of these groups.
These objects will be achieved with a telephone network system that offers centrex services and a method of offering a network system having centrex services.